In order to investigate factors of importance for successful reproduction of sea trout, Salmo trutta L., in brackish water, fertilization and incubation of eggs were conducted at different salinities at about 4[degree sign]C. In addition, duration of spermatozoan motility at different salinities was quantified by observations on sperm from 11 males. The results indicate that fertilization is possible at salinities characteristic of the waters around the Baltic island of Gotland, Sweden (6.7 psu) but the ensuing egg development is inhibited at salinities exceeding 4.0 and no hatching was observed in ambient Baltic Sea water (BSw). Sea trout have been observed to spawn along the Gotland shore but the outcome of this effort, in terms of recruitment to the sea trout population, must be regarded as non-existent.